A functional and histochemical characterization of motor units in a heterogeneous muscle (flexor digitorum longus) of the cat

Abstract

AbstractThis paper reports an attempt to differentiate motor units of several types both physiologically and histochemically.The average characteristics of the motor units in the flexor digitorum longus (FDL), flexor hallucis longus (FHL) and soleus muscles of cats with one chronically deafferented hind limb were studied. The average maximal tension per motor unit was calculated to be 14.5 g for FDL, 25.8 g for FHL and 19.7 g for soleus. In general, the average size of the motor units in each muscle increased with the weight of the animal. The total number of motor units to each muscle and the diameters of all efferent nerve fibers were determined in histological cross‐sections of the deafferented nerve.The FDL muscle was selected for study of individual units isolated from the ventral roots of 15 cats. The conduction velocities of 108 axons ranged from 71.3 to 114.3 m/sec. These units developed maximal isometric tensions ranging from 0.4 to 100.0 g, with an average of 9.6 g. The frequency distribution of maximal tensions was markedly skewed, with 83 units developing less than 10 g. The twitch tensions of 90 units averaged 0.9 g, with a range of 0.1 to 10.0 g. All units in which no twitch was detected developed less than 5 g of maximal tension. The small size of the units and the limitations of the equipment were thought responsible for the occasional failure to record a twitch. The contraction times of 85 units fell between 13 and 81 msec, with a modal class of 30 to 35 msec. During continuous stimulation at 100/sec, large units usually proved more susceptible to fatigue than small units. Regression analyses of conduction velocity versus maximal tetanic tension revealed that large units (> 19 g) belonged to a different population than units developing less than 19 g of maximal tension. In addition, the findings suggest that small units may perhaps be divided into a slow and fast group according to their contraction times.Mitochondrial adenosine triphosphatase activity was demonstrated in cross‐sections of FDL muscle fibers. A correlation between histochemically distinguishable fiber types and functionally distinguishable motor units is proposed.